Hybrid development of electrostatic latent image

ABSTRACT

An apparatus in which a cloud of toner particles is produced in a development zone for rendering viewable an electrostatic latent image. Subsequently, the unused toner particles are formed into a cloud of particles in a retrieval zone for return to the developer mix.

United States Patent 11 1 [111 3,799,113

Whited Mar. 26, 1974 1 HYBRID DEVELOPMENT OF 3,412,710 11/1968 Robinson 118/637 ELECTROSTATIC LATENT IMAGE 3,424,131 1/1969 Aser et a1. 118/637 3,470,009 9/1969 Gundlach 118/637 x lnventorr Charles Whited, R st Y. 3,685,488 8/1972 Stover 118/637 3,648,658 3/1972 Weiler 118/637 [73] Assgnee xerox Stamfmd' 3,695,224 10/1972 Royka 118/637 Conn.

[22] Fil d. Ju 1972 Primary ExaminerMervin Stein [21] Appl. No.: 266,937

[57] ABSTRACT 52 U.S. c1 118/637, 117/175, 118/636 An apparatus in which a cloud of toner Particles is 51 1111. c1 G03g 13/08 Produced in a development zone for rendering view- [58] Field 61 Search 118/636, 637, DIG. 24 able an electrostatic latent image. Subsequently, the

unused toner particles are formed into a cloud of par- [56] References Ci d ticles' in a retrieval zone for return to the developer UNITED STATES PATENTS 3,011,474 12/1961 Ulrich 118/637 10 Claims, 3 Drawing Figures HYBRID DEVELOPMENT OF ELECTROSTATIC LATENT IMAGE BACKGROUND OF THE INVENTION This invention relates generally to electrostatographic printing, and more particularly concerns an apparatus for developing an electrostatic latent image.

Electrostatographic printing includes both electrographic printing and electrophotographic printing. Both of these processes are quite similar to one another, i.e. an electrostatic charge pattern or latent image is recorded on an image bearing member which corresponds to the original document to be reproduced. In the process of electrophotography, as disclosed in US. Pat. No. 2,297,691 issued to Carlson in 1942, a photosensitive element having a photoconductive insulating layer is charged to a substantially uniform potential. The charged photoconductive surface is then exposed to a light image of the original document. This light image selectively dissipates the charge in the irradiated areas and creates an electrostatic latent image on the photoconductive surface. Electrographic printing creates an electrostatic latent image corresponding to the original document to be reproduced without the use of photosensitive materials.

Whatever method is employed in the formation of an electrostatic latent image, a viewable record thereof is usually produced by depositing toner particles thereon, i.e., development. Development may be achieved by bringing the latent image into contact with a developer mix. Typical developer mixes employed in the art generally comprise toner particles, such as colored thermoplastic particles, which electrostatically adhere to coarser carrier granules, such as ferromagnetic granules.

Various developing systems are well-known in the art and include, amongst others, cascade development, magnetic brush development, powder cloud development and liquid development. The present invention employs a hybrid system combining cascade and powder cloud development. One such hybrid development system is disclosed in United States Pat. No. 3,470,009 issued to Gundlach in 1969. The development system described therein includes a bucket conveyor for transporting a developer mix from a sump to an upper region for discharge thereat. At the upper region, the developer mix cascades in a downwardly direction between an apertured screen and electrode. Toner particles are separated from the carrier granules and move through the screen which prevents the passage of carrier granules therethrough. A quantity or cloud of toner particles is moved into the space adjacent the latent image. The electrostatic field created by the latent image attracts the toner particles thereto to render the image viewable.

This type of hybrid development system has been found to be an excellent technique for continuous tone reproduction and other solid area coverage. However, no provision is made in the apparatus heretofore utilized for the reclamation of unused toner particles. As a result thereof, particles frequently collect on wall surfaces and in voids, thereby deleteriously effecting the clarity of a copy being reproduced.

Accordingly, it is the primary object of the present invention to improve the development apparatus utilized in electrostatographic printing. SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention, there is provided an apparatus for producing a viewable image of an elecrostatic latent image re corded on an image bearing member.

In the preferred embodiment, the apparatus includes a guide member, a pair of opposed, spaced, side walls defining a chute therebetween for feeding a developer mix comprising carrier granules and toner particles therein, and means for applying a voltage to the pair of side walls. The pair of side walls is disposed spaced from and intermediate the image bearing member and guide member. In this way, a development zone is formed between the image bearing member and the first side wall opposed therefrom, and a retrieval zone between the guide member and the second side wall opposed therefrom. Each of the pair of side walls have a perforated region extending substantially coextensive with the latent image and adapted to prevent carrier granules from passing therethrough while permitting toner particles to pass therethrough.

Further, in accordance with the present invention, the voltage applying means initially electrically biases the first side wall to a potential above the second side wall. As a consequence thereof, toner particles are moved through the perforated region in the first wall forming a cloud of toner particles in the development zone for developing the latent image. After developing the latent image, the voltage applying means electrically biases the second side wall to a potential above the first side wall. This results in the unused toner particles moving through the perforated region in the second side wall forming a cloud of unused toner particles in the retrieval zone for intermingling with the developer mix.

BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic sectional view of an electrophotographic printing machine;

FIG. 2 is an enlarged, fragmentary, sectional view illustrating the development apparatus used in the FIG. 1 printing machine; and

FIG. 3 is an enlarged, fragmentary, sectional view depicting the apparatus in the FIG. 2 development apparatus for generating a cloud of toner particles.

While the present invention will be described in connection with a preferred embodiment and procedure, it will be understood that it is not intended to limit the invention to that embodiment and procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION With continued reference to the drawings, wherein like reference numerals have been used throughout to designate like elements, FIG. 1 illustrates one embodiment of an electrophotographic printing machine incorporating the present invention therein. Generally, an electrophotographic machine projects a light image ofa document to be reproduced onto a sensitized photoconductive surface to form an electrostatic latent image thereof. This latent image is developed by toner particles deposited thereon to form a powder image thereof. Subsequently, the powder image is electrostatically transferred to a sheet or web of final support material. The powder image is, thereafter, fixed to the support material by a suitable fusing device which causes it to permanently adhere thereto.

The printing machine depicted in FIG. I employs a photoconductive or image bearing member which, preferably, is a rotatably mounted drum 10 having a photoconductive surface 12 thereon. Photoconductive surface 12 is made from an insulating material such as vitreous selenium. Drum 110 is rotatably about its axis, in a direction indicated by arrow 14, by suitable drive means, not shown. As drum I rotates in the direction of arrow M it moves photoconductive surface 12 sequentially through a series of processing stations.

Photoconductive surface 12 passes first through charging station A which has positioned thereat a corona generating device, indicated generally at 16, extending transversely across photoconductive surface 12. The corona generating device 16 charges drum surface I2 to a relatively high and substantially uniform potential.

The charged drum surface is next rotated to exposure station B which includes an exposure mechanism 18 for irradiating the charged photoconductive surface 12 with a light image of the original document to be reproduced. In this manner, portions of photoconductive surface I2 are selectively discharged by the light image. While the invention has been described in connection with a light image forming an electrostatic latent image, one skilled in the art will appreciate that the invention is not necessarily so limited and that other means for forming images on ordinary insulating surfaces are known in the art and may be used in lieu of the present one depicted in FIG. 1.

After the electrostatic latent image is recorded on photoconductive surface 112, drum is rotated to development station C which deposits toner particles thereon producing a viewable image. The development unit, indicated generally at 20, forms a powder cloud of toner particles which is electrostatically attracted to the latent image recorded on photoconductive surface 12. Image development will be further described hereinafter in detail.

After development, the now visible image is moved to transfer station D where the powder image is transferred to a web of final support material 22 such as plain paper, amongst others. Support material 22 is positioned between guide rolls 2 1 and 26. A second corona generating device 28 electrostatically tacks support material 22 against photoconductive surface 22 and transfers the toner powder image therefrom to support material 22. The Web of support material 22 is unwound from supply roll 30 and rewound onto takeup roll 32.

After the transfer operation and prior to being wound onto takeup roll 32, support material enters fixing station E where a fuser, indicated generally at 34, is positioned to heat the powder image and thus fix it permanently to support material 22.

The last processing station, in the direction of rotation of drum It) as indicated by arrow M, is cleaning station G. After each transfer operation a rotatably mounted fibrous brush 36 contacts photoconductive surface 12 to remove residual toner particles remaining thereon.

Referring now to FIG. 2, there is shown in detail, development unit 20. Development unit 20 is depicted in a sectional elevational view to more clearly indicate the various components included therein. The principle components of developer unit 26 are developer housing 38, side walls 48 and 56, respectively, guide member 42, first conveyor means 44, and second conveyor means 46. Side walls 43 and 50 are spaced from and opposed to one another. First side wall 68 includes a perforated region or screen 52 spaced from and extending substantially co-extensive with the electrostatic latent image recorded on photoconductive surface 12. Second side wall 50 also includes a perforated region or screen 543. Second side wall 51) is intermediate first side wall 52 and guide member '52. Screen 54 also extends coextensive with the electrostatic latent image recorded on photoconductive surface 12. Screens 52 and 54 may be made from any suitable conductive ma terial, for example, fine conductive screens. The aperture or mesh size is dependent upon having openings sufficiently large to permit toner particles to pass therethrough without blocking the openings, while preventing carrier granules from passing therethrough and also preventing distortion of the electrical field. Preferably, guide member 42 is made from an insulating material, e.g., bakelite.

Image development is effected by developer mix 56 being supplied to feeding chute 62 defined by side walls l8 and 50, respectively. First conveyor means M is a bucket conveyor arranged to move the developer mix from a first region 56 in the lower portion of developer housing 36 for discharge thereat into chute 62. Dcveloper mix 56 cascades in a downwardly direction in chute 62, as indicated by arrow 64. A suitable voltage means 66 applies a DC potential ranging from about 150 to about 300 volts to screen 62, electrically biasing screen 52 above screen 56. As developer mix 56 descends in a downwardly direction, as indicated by arrow 64, toner particles are stripped from the carrier granules and pass through screen 52 into development zone 66. The cloud of toner particles in development zone 62 is adjacent to the electrostatic latent image recorded on photoconductive surface 12. The particles are attracted thereto forming a toner powder image corresponding to the latent image recorded thereon.

Turning once again to FIG. 2, after the electrostatic latent image on photoconductive surface I2 has been developed, the voltage being applied to screen 52 is removed therefrom, and voltage means 70 applies a DC potential of about to about 300 volts to screen 54 electrically biasing screen 54 above screen 52. Simultaneously therewith, second conveyor means 46, which is a bucket conveyor, moves developer mix 56 from the lower region 56 of developer housing 36 to the third region or upper region 72 where developer mix descends in a downwardly direction, as indicated by arrow 76, into retrieval zone 74 defined by screen 54 and guide member 42. When the DC potential is applied to screen 56, the unused toner particles pass through screen 54 into retrieval zone 74 where they interact with the developer mix descending from conveyor $6. In this manner the residual or unused toner particles are returned to the lower region of developer housing 36 for subsequent reuse.

The potential applied to screens 52 and 54 should be of sufficient magnitude to overcome the triboelectric attraction between the carrier granules and toner particles and move the toner particles in the appropriate direction. For example, when screen 52 is electrically biased above screen 54 the particles should move toward photoconductive surface 12, while when screen 54 is electrically biased above screen 52, the particles should move toward guide member 42. It is evident that the required voltage level will be dependent upon the spacing between screens 52 and 54 as well as the required particle acceleration. As so far described separate voltage supplies 66 and 70 electrically bias the corresponding screens 52 and 54 to the appropriate potential level at the appropriate time. By way of example voltage supply 66 biases screen 52 during development and is, therefore, de-energizcd when voltage supply 70 biases screen 54 during reclamation of unused toner particles. However, it is apparent that in lieu of a pair of voltage supplies, a single voltage supply may be utilized on conjunction with suitable switching circuitry for electrically biasing each screen at the correct time during the machine cycle.

Turning now to FIG. 3, there is shown in detail the relationship between the screen members, guide member and photoconductive surface. As shown therein, development is initiated when developer mix 56 is descending in chute 62 and passing between opposed, spaced screens 52 and 54. At this time screen 52 is raised to a DC potential level above that of screen 54. In this manner toner particles are stripped from the carrier granules and pass through the apertures in screen 52 into development zone 68. In development zone 68, the toner powder cloud is adjacent to and in contact with the electrostatic latent image recorded on photoconductive surface 12. The electrostatic latent image attracts toner particles thereto so as to form a toner powder image of the original document to be reproduced on photoconductive surface 12. After the toner powder image has been formed on photoconductive surface 12, the residual or unused toner particles are reclaimed by removing the DC potential from screen 52 and raising screen 54 to a DC potential level above that of screen 52. This causes the unused toner particles to migrate through screen 54 into retrieval zone 74. in retrieval zone 74, the unused toner particles interact with the developer mix descending thereat and are returned to the sump of developer housing 38 for subsequent reuse.

ln recapitulation, it is apparent that the present invention improves the development apparatus of an electrostatographic printing machine by combining cascade development with toner powder cloud development to form a cloud of toner particles adjacent the photoconductive surface for developing the electrostatic latent image recorded thereon. Moreover, the residual or unused toner particles are subsequently reclaimed and returned to the sump of the developer housing for reuse thereafter.

It is, therefore, evident that there has been provided in accordance with this invention, an apparatus for developing an electrostatic latent image recorded on an image bearing member that fully satisfies the objects, aims and advantages set forth above. While this invention has been described in conjunction with specific embodiments and methods thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

What is claimed is:

1. An apparatus for rendering visible an electrostatic latent image recorded on an image bearing member, including:

a guide member;

a pair of opposed, spaced, side walls defining a chute therebetween for feeding a developer mix comprising carrier granules and toner particles therethrough, said pair of side walls being disposed spaced from and intermediate said guide member and the image bearing member forming a development zone between the image bearing member and said first side wall opposed therefrom and a retrieval zone between said guide member and said second side wall opposed therefrom, said pair of side walls each having a perforated region extending substantially co-extensive with the latent image and adapted to prevent carrier granules from passing therethrough while permitting toner particles to pass therethrough; and

means for applying a voltage to said pair of side walls so that initially said first side wall is electrically biased to a potential above said second side wall to move toner particles through the perforated region in said first side wall forming a cloud of toner particles in the development zone for developing the latent image, and, after developing the latent image, said second side wall is electrically biased to a potential above said first side wall to move unused toner particles through the perforated region in said second side wall forming a cloud of unused toner particles in the retrieval zone for intermingling with developer mix passing therethrough.

2. An apparatus as recited in claim 1, further including:

first conveyor means for moving the developer mix from a first region to a second region for discharg into the feed chute; and

second conveyor means for moving the developer mix from the first region to a third region for discharge into the retrieval zone where the developer mix interacts with the cloud of unused toner particles formed therein to return therewith to the first region for subsequent reuse.

3. An apparatus as recited in claim 2, wherein said first side wall is electrically biased to a DC potential ranging from about volts to about 300 volts above the potential of said second side wall.

4. An apparatus as recited in claim 2, wherein said second side wall is electrically biased to a DC potential ranging from about 150 volts to about 300 volts above the potential of said first side wall.

5. An apparatus as recited in claim 2, further including means for imparting relative movement between the image bearing member and the cloud of toner particles moving toward the electrostatic latent image in the development zone.

6. An improved electrophotographic printing machine of the type wherein a corona generating device charges a photoconductive surface to a substantially uniform charge potential, and wherein an exposure mechanism produces a light image which selectively discharges the charged photoconductive surface forming an electrostatic latent image of an original docusaid second side wall forming a cloud of unused ment to be reproduced, wherein the improvement intoner particles in the retrieval zone for intermin eludes: gling with developer mix passing therethrough.

' guidfi member; 7. A machine as recited in claim 6, further including; a pair of opposed, spaced, side walls defining a chute 5 therebctwee" for feeding a developer mix P first conveyor means for moving the developer mix ing Carri granules and toner Particles there from a first region to a second region for discharge through, said pair of side walls being disposed into the feed Chute; and

spaced from and intermediate said guide member and the image bearing member forming a development zone between the image bearing member and said first side wall opposed therefrom and a retrieval zone between said guide member and said second side wall opposed therefrom, said pair of side walls each having a perforated region extending substantially co-extensive with the latent image and adapted to prevent carrier granules from passing therethrough while permitting toner particles to I pass therethrough; and tentlal of sald second sidewall. means for applying a voltage to said pair of side walls 20 as 'f m j whcrem F so that initially said first side wall is electrically bi- 0nd elecmcally blased a DC P aged to a potential above Said Second Side n to ranging from about 150 volts to about 300 volts above move toner particles through the perforated region the Potential of Said first Side Wallin said first side wall forming a cloud of toner parti- A machine recited in Claim 1 further including l i th development zone f d l i h l means for imparting relative movement between the tent imag d ft d l i h l t i photoconductive surface and the cloud of toner partisaid second side wall is electrically biased to a pocles moving toward the electrostatic latent image in the tential above said first side wall to move unused development zone. toner particles through the perforated region in second conveyor means for moving the developer mix from the first region to a third region for discharge into the retrieval zone where the developer mix interacts with the cloud of unused toner particles formed therein to return therewith to the first region for subsequent reuse. 8. A machine as recited in claim 7 wherein said first side wall is electrically biased to a DC potential ranging from about 150 volts to about 300 volts above the po- 

1. An apparatus for rendering visible an electrostatic latent image recorded on an image bearing member, including: a guide member; a pair of opposed, spaced, side walls defining a chute therebetween for feeding a developer mix comprising carrier granules and toner particles therethrough, said pair of side walls being disposed spaced from and intermediate said guide member and the image bearing member forming a development zone between the image bearing member and said first side wall opposed therefrom and a retrieval zone between said guide member and said second side wall opposed therefrom, said pair of side walls each having a perforated region extending substantially co-extensive with the latent image and adapted to prevent carrier granules from passing therethrough while permitting toner particles to pass therethrough; and means for applying a voltage to said pair of side walls so that initially said first side wall is electrically biased to a potential above said second side wall to move toner particles through the perforated region in said first side wall forming a cloud of toner particles in the development zone for developing the latent image, and, after developing the latent image, said second side wall is electrically biased to a potential above said first side wall to move unused toner particles through the perforated region in said second side wall forming a cloud of unused toner particles in the retrieval zone for intermingling with developer mix passing therethrough.
 2. An apparatus as recited in claim 1, further including: first conveyor means for moving the developer mix from a first region to a second region for discharge into the feed chute; and second conveyor means for moving the developer mix from the first region to a third region for discharge into the retrieval zone where the developer mix interacts with the cloud of unused toner particles formed therein to return therewith to the first region for subsequent reuse.
 3. An apparatus as recited in claim 2, wherein said first side wall is electrically biased to a DC potential ranging from about 150 volts to aBout 300 volts above the potential of said second side wall.
 4. An apparatus as recited in claim 2, wherein said second side wall is electrically biased to a DC potential ranging from about 150 volts to about 300 volts above the potential of said first side wall.
 5. An apparatus as recited in claim 2, further including means for imparting relative movement between the image bearing member and the cloud of toner particles moving toward the electrostatic latent image in the development zone.
 6. An improved electrophotographic printing machine of the type wherein a corona generating device charges a photoconductive surface to a substantially uniform charge potential, and wherein an exposure mechanism produces a light image which selectively discharges the charged photoconductive surface forming an electrostatic latent image of an original document to be reproduced, wherein the improvement includes: a guide member; a pair of opposed, spaced, side walls defining a chute therebetween for feeding a developer mix comprising carrier granules and toner particles therethrough, said pair of side walls being disposed spaced from and intermediate said guide member and the image bearing member forming a development zone between the image bearing member and said first side wall opposed therefrom and a retrieval zone between said guide member and said second side wall opposed therefrom, said pair of side walls each having a perforated region extending substantially co-extensive with the latent image and adapted to prevent carrier granules from passing therethrough while permitting toner particles to pass therethrough; and means for applying a voltage to said pair of side walls so that initially said first side wall is electrically biased to a potential above said second side wall to move toner particles through the perforated region in said first side wall forming a cloud of toner particles in the development zone for developing the latent image, and after developing the latent image, said second side wall is electrically biased to a potential above said first side wall to move unused toner particles through the perforated region in said second side wall forming a cloud of unused toner particles in the retrieval zone for intermingling with developer mix passing therethrough.
 7. A machine as recited in claim 6, further including; first conveyor means for moving the developer mix from a first region to a second region for discharge into the feed chute; and second conveyor means for moving the developer mix from the first region to a third region for discharge into the retrieval zone where the developer mix interacts with the cloud of unused toner particles formed therein to return therewith to the first region for subsequent reuse.
 8. A machine as recited in claim 7 wherein said first side wall is electrically biased to a DC potential ranging from about 150 volts to about 300 volts above the potential of said second side wall.
 9. A machine as recited in claim 7, wherein said second side wall is electrically biased to a DC potential ranging from about 150 volts to about 300 volts above the potential of said first side wall.
 10. A machine as recited in claim 7, further including means for imparting relative movement between the photoconductive surface and the cloud of toner particles moving toward the electrostatic latent image in the development zone. 